Biochar prepared from castor oil cake at different temperatures: A voltammetric study applied for Pb2+, Cd2+ and Cu2+ ions preconcentration

Journal of Hazardous Materials 318 (2016) 526–532

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Biochar prepared from castor oil cake at different temperatures: A voltammetric study applied for Pb2+ , Cd2+ and Cu2+ ions preconcentration Cristiane Kalinke a , Antonio Sálvio Mangrich b,c , Luiz H. Marcolino-Junior a , Márcio F. Bergamini a,∗ a Laboratório de Sensores Eletroquímicos (LabSensE), Departamento de Química, Universidade Federal do Paraná (UFPR), CEP 81.531-980 Curitiba, PR, Brazil b Laboratório de Química de Húmus e Fertilizantes, Departamento de Química, Universidade Federal do Paraná (UFPR), CEP 81.531-980 Curitiba, PR, Brazil c Instituto Nacional de Ciência e Tecnologia de Energia e Ambiente (INCT E&A/CNPq), Brazil

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Effect of temperature of pyrolysis in the adsorption properties of biochar. application of biochar obtained from castor cake oil under different temperature pyrolysis. • Simple and feasible voltammetric procedure for evaluation of adsorption of Pb(II), Cd(II) and Cu(II) ions by biochar. • Carbon paste electrodes for evaluation of biochar ability in the preconcentration of electroactive cations.

• Electroanalytical

a r t i c l e

i n f o

Article history: Received 18 May 2016 Received in revised form 24 June 2016 Accepted 18 July 2016 Available online 19 July 2016 Keywords: Biochar Pyrolysis temperature Carbon paste electrode Voltammetric techniques

a b s t r a c t Biochar is a carbonaceous material similar produced by pyrolysis of biomass under oxygen-limited conditions. Pyrolysis temperature is an important parameter that can alters biochar characteristics (e.g. surface area, pore size distribution and surface functional groups) and affects it efficacy for adsorption of several probes. In this work, biochar samples have been prepared from castor oil cake using different temperatures of pyrolysis (200–600 ◦ C). For the first time, a voltammetric procedure based on carbon paste modified electrode (CPME) was used to investigate the effect of temperature of pyrolysis on the adsorptive characteristics of biochar for Pb(II), Cd(II) and Cu(II) ions. Besides the electrochemical techniques, several characterizations have been performed to evaluate the physicochemical properties of biochar in function of the increase of the pyrolysis temperature. Results suggest that biochar pyrolized at 400 ◦ C

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (M.F. Bergamini). http://dx.doi.org/10.1016/j.jhazmat.2016.07.041 0304-3894/© 2016 Elsevier B.V. All rights reserved.

C. Kalinke et al. / Journal of Hazardous Materials 318 (2016) 526–532

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(BC400) showed a better potential for ions adsorption. The CPME modified with BC400 showed better relative current signal with adsorption affinity: Pb(II) > Cd(II) > Cu(II). Kinetic studies revealed that the pseudo-second order model describes more accurately the adsorption process suggesting that the surface reactions control the adsorption rate. Values found for amount adsorbed were 15.94 ± 0.09; 4.29 ± 0.13 and 2.38 ± 0.39 ␮g g−1 for Pb(II), Cd(II) and Cu(II) ions, respectively. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Biochar can be defined as a carbonaceous material similar to charcoal produced by pyrolysis of biomass under oxygen-limited conditions [1,2]. It is a porous grained material, with high carbon content and largely resistant to decomposition [3]. The peripheral structure (surface) of biochar exhibits a high amount of chemical groups, which can be used as a sorbent for different compounds, such as metallic ions and organic molecules [4–6]. Moreover, it has an inert internal structure, which can act in carbon sequestration as well as organic soil conditioner [7,8]. In this sense, several works have been reported the use of biochar, obtained from several feedstock and prepared using different pyrolysis conditions, for soil amendment improving agricultural production by nutrient retention, sorbing of organic/inorganic pollutants besides that it is a recalcitrant carbon stock [9–11]. In general, experimental conditions used for biochar production are low temperatures (<700 ◦ C) [1], slow pyrolysis with heating rates of 1–100 ◦ C min−1 and long residence times (minutes–hours) [12]. Pyrolysis conditions and features of the feedstock can alter the physical and chemical properties of the final product (biochar) such as concentrations of elemental constituents, density, pore size distribution and surface characteristics, among others [13–15]. Different feedstock, mainly of ligno-cellulosic materials, such as peanut hull [13], pine wood [16], wheat straw [17], corn stover [18] and several others [9] have been used for biochar preparation. Although, biomass composition presents different thermal behavior, the temperature used in the pyrolysis process can be considered the most important condition and it can influence on the final product characteristics [1,19]. Biochar is used for soil amendment or remediation of contaminated soils and waters by organic and/or inorganic species [20–27], for bio-oils hydrocracking and hydrodeoxygenation upgrading processes [28,29], for improving air quality [30,31] and for developing catalysts [32,33]. The highly functionalized surface of biochar promotes an elevate sorption capacity when compared to other adsorbent materials such as activated carbon [5]. Besides of agricultural applications, features of biochar surface can be explored in the development of electrochemical devices. Suguihiro et al. [34] were pioneers in the use of biochar for development of electrochemical sensors for determination of metallic cations. The use of biochar prepared in a narrow temperature range (300–350 ◦ C) as electrode modifier promoted high selectivity and sensitivity for preconcentration and determination of cadmium and lead ions in industrial effluents samples. At the same way, Oliveira et al. [35] related the determination of Cu(II) ions on spirit drinks samples and Kalinke et al. [36] described a method for paraquat determination in coconut water and natural water samples. Moreover, the use of biochar as support material for preparation of mercury, antimony and bismuth nanostructures was reported. Those methodologies were applied for determination of zinc ions in pharmaceutical formulations [37], paraquat in fruit juices [38] and lead ions in ceramic dishes [39], respectively. Although biochar has called attention for development of electroanalytical methods, there are no information about a systematic study using

a voltammetric technique for evaluation of biochar adsorption capacity. Based on the above-cited works, which have explored successfully the quantitative aspects in the use of biochar on electroanalytical applications, the main goal of the present work is the use of voltammetric tools to verify the effect of pyrolysis temperature of biochar samples prepared from castor oil cake and develop a protocol to investigate the mechanisms involved in the interaction between biochar surface and metallic ions (Pb(II), Cd(II) and Cu(II)) by using of kinetic models of pseudo-first and pseudosecond order. 2. Materials and methods 2.1. Apparatus Differential Pulse Adsorptive Stripping Voltammograms (DPAdSV) measurements were performed in a potentiostat/galvanostat ␮AUTOLAB Type III (EcoChemie) connected to a microcomputer controlled by software (NOVA1.10.4® ) for data acquisition and experimental control. All the voltammetric measurements were carried out in a 10 mL glass cell at 25 ◦ C, with a three-electrode configuration: carbon paste modified electrode as the working electrode, Ag/AgCl KCl 3.0 mol L−1 as the reference electrode and platinum wire auxiliary electrode. 2.2. Reagents and solutions All the solutions were prepared with water purified in a Millipore Milli-Q system. All the chemicals were of analytical grade and were used without further purification. A stock solution containing 1000 mg L−1 of ions (Merck) was used and solutions containing different concentrations of Pb(II), Cd(II) and Cu(II) ions were prepared by dilution. 2.3. Biochar preparation and characterization Biochar samples were obtained from biomass pyrolysis, utilizing castor oil cake feedstock. First, the material was sieved to obtain particle size between 177 to 420 ␮m, and then was subjected to the pyrolysis process in a EDG FT-40 tubular furnace, with conditions programmed of heating rate of 5 ◦ C min−1 , during 60 min and final temperatures ranging between 200 and 600 ◦ C. The material characterization was realized using thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC), performed on a Netzsch STA 449F3, under nitrogen flow, until 1000 ◦ C. Information about functional groups present at surface of the biochar was provided by a FTIR Bomem MB100 spectrometer recording the spectrum scope from 4000 to 400 cm−1 with a resolution of 4 cm−1 and Boehm titrations [40] using a Metrohm 780 pH meter. Elemental composition was determined using a PerkinElmer 2400 Series II CHNS/O Elemental Analyzer. Surface area and pore size distribution of samples was performed on a Quantachrome porosimeter, New 1200 model, with sorption of nitrogen gas, using the B.E.T. method (Brunauer, Emmett and Teller).

528

C. Kalinke et al. / Journal of Hazardous Materials 318 (2016) 526–532 20 %

B

A

TGA DSC

100

40 20

Yield Weight loss

200

400 Temperature / ºC

600

60 -0.4

40 -0.6

318 ºC

20 0

-0.8

472 ºC

300 600 Temperature / ºC

900

Fig. 1. Yield and weight loss for biochar samples prepared with pyrolysis at different temperatures (A). Thermogravimetric (TGA) and Differential Scanning Calorimetric (DSC) curves obtained for biochar sample prepared at 200 ◦ C (BC200), in nitrogen atmosphere (B).

Absorbance / %

60

-0.2

-1

Weight lost / %

BM

80

DSC / mV mg

Weight / %

80

0.0

BC200

BC300

BC400

BC500

BC600

3600

2.4. Electrode construction Carbon paste modified electrode was prepared by mixing of mineral oil (25% (w/w)) (Nujol® ), graphite powder (50% (w/w)) (Aldrich® ), and modifier (biochar sample at different temperatures, biomass not pyrolyzed and actived carbon) (25% (w/w)). CPE was prepared with mineral oil (25% (w/w)) and graphite powder (75% (w/w)). The components were homogenized manually with pestle and mortar. The composite obtained was packed into support of electrodes, consisting of a PVC tube (int = 3 mm), containing with a copper rod, used as electrical contact of the electrode and paste compression. 2.5. Voltammetric procedures The potentiality of modified electrodes for preconcentration of Pb(II), Cd(II) and Cu(II) ions was investigated using differential pulse adsorptive stripping voltammetry (DPAdSV) procedure. The analytical procedure could be summarized in four steps: (1) Preconcentration: ions had been preconcentrated at electrode surface in 0.10 mol L−1 acetate solution pH 7.0, in open circuit potential under controlled stirring, for 5 min; (2) Reduction of the adsorbed ions: the electrode was removed from the preconcentration container, it was gently washed and placed in the electrochemical cell containing 10 mL of 0.10 mol L−1 acetate buffer solution, pH 5.0. A potential of −1.0 V was applied during 120 s in order to promote the electrochemical reduction of the adsorbed ions at electrode surface; (3) Stripping: measurements of DPAdSV were registered by application of a sweep of potential to positive direction; (4) Cleaning: the electrode was cleaned by putting it in 0.10 mol L−1 H2 SO4 solution, under constant agitation for 5 min. 3. Results and discussion 3.1. Preparation and general characterization of biochar Biochar samples were obtained using temperature values varying from 200 to 600 ◦ C. Fig. 1A shows the yield and weight loss for biochar samples prepared at different pyrolysis temperatures. Results revealed that higher temperatures, employed in the pyrolysis process, promote more significant degradation of biomass leading to a low yield of biochar. The most significant changes that occurring can be described as: dehydration, pyrolysis and carbonization [41–43]. At temperatures around 250 ◦ C, the primary changes in the feedstock begins to occur: dehydration of biomass and depolymerization of cellulose, and only a small mass variation is observed in this region. Above 250 ◦ C until the temperature of 600 ◦ C, pyrolysis occurs more significantly, involving different stages. Between 250 and 350 ◦ C there is a complete depolymeriza-

3000

2400

1800

Wavenumber / cm

1200

600

-1

Fig. 2. FTIR spectrum for samples of biomass (BM) and biochar (BC) at different temperatures of pyrolysis.

tion of cellulose (hydrolysis) which promotes a significant weight loss and the appearance of an essentially amorphous carbon matrix. At temperature values around 330 ◦ C, the first aromatic carbon signs were observed, and about 350 ◦ C, polyaromatic graphene sheets begin to grow [44–46]. The most pronounced decrease in yield was observed at temperatures between 300 and 400 ◦ C, which are associated with the degradation of biomass components, such as cellulose, hemicellulose and lignin. These compounds exhibit distinctive thermal behavior depending on the heating rate of the pyrolysis process [47]. At low heating rates, such as 5 ◦ C min−1 , cellulose decomposition begins at temperatures above 300 ◦ C, and the maximum decomposition observed at 345 ◦ C [48]. At temperatures above 600 ◦ C begins the carbonization and graphene sheets continue to grow [44]. In order to verify the thermal behavior of samples prepared at different temperatures of pyrolysis, measurements of TGA and DSC for biochar sample pyrolyzed at 200 ◦ C (BC200) were performed (Fig. 1B). Significant weight loss can be observed at two different temperatures, the first at around 318 ◦ C with 52% of lost weight, while the second corresponds to 33% by weight, and occurred at 472 ◦ C. The decomposition of hemicelluloses may begin at temperatures around 300 ◦ C, while the mass decrease at 412 ◦ C corresponds to cellulose decomposition [49]. This degradation can be confirmed by DSC curve, through the existence of an exothermic process. The biomass and other biochar samples were submitted to TGA and DSC analysis, the thermogravimetric curves (Supplementary data, Fig. S1) demonstrated that weight loss is concordant with increase in the pyrolysis temperature used for biochar production and can be related to the degradation of compounds such as cellulose, hemicellulose and lignin still present in the samples [42]. In order to obtain information about functional groups present at samples surfaces, FTIR spectrum for the biomass (BM) and biochar samples (BC) produced at different temperatures were performed and are shown in Fig. 2. FTIR spectrum obtained for biochar samples are characterized by several major bands such as OH bond stretch around 3400 cm−1 [50], this is observed in samples from BM and BC obtained at temperatures below 400 ◦ C, this behavior can be attributed to the dehydration of biomass with increasing pyrolysis temperature [51]. Vibrations between 3000 and 2800 cm−1 corresponding to the CH stretching, degraded with increasing temperature [52]. These results can be related to those obtained with the TG analysis of biochar samples, which suggests that the hemicellulose and cellulose are degraded more markedly at temperatures above

C. Kalinke et al. / Journal of Hazardous Materials 318 (2016) 526–532 Table 1 Percentage weight/weight (% wt) of CHNS-O obtained for the samples by biomass, biochar, activated charcoal (AC) and graphite.

BC200

% wt C

BM BC200 BC300 BC400 BC500 BC600 GRAPH AC

1.6

H/C Atomic Ratio

Sample

45.7 46.6 48.3 49.6 50.7 52.2 42.0 42.1

H 5.71 5.69 5.24 4.31 3.76 3.38 4.62 4.74

N

S

Oa

10.9 11.6 11.5 12.0 11.8 12.3

14.5 15.0 13.6 14.3 14.0 14.4

23.1 21.1 21.4 19.9 19.6 17.7

b

b

b

b

18.1

b

Elemental composition values are reported based on dry weight of sample. a Calculated by subtracting. b Not detected.

400 ◦ C [43]. In the region 2400–2200 cm−1 has been related to the peak absorption of CO2 . There is a band at 1740 cm−1 with signal maximum for temperature of 200 ◦ C which can be attributed to group C O of ester. When higher pyrolysis temperatures are used is possible to observe a decreased and slight shifted to 1700 cm−1 reference to carboxyl, aldehydes, ketones and esters [52]. Higher absorbent IR spectrum regions correspond substantially to the aromatic vibrations, the band near 1600 cm−1 corresponds to aromatic C C stretching [53]. In 1500 and 1400 cm−1 band can be observed regarding CH deformations, and 1440 cm−1 corresponding to a region where various functional groups can produce absorption. Intensities of the bands between 1500 and 1400 cm−1 increased suggesting that they are related mainly with aromatic components probably the C C vibration [54]. Signals between 1400 and 1200 cm−1 may correspond to the presence of OH− groups present in phenols and carboxylic acids. A medium intensity band is between 1000 and 900 cm−1 , which is characteristic of C O C asymmetric stretching, from cellulosic matters [55]. The increase in pyrolysis temperature affected significantly the degradation of aliphatic carbon groups present on the samples surface, resulting in decrease intensity of these bands in the IR spectrum. Comparing the FTIR spectrum of biochar sample with activated carbon and graphite (Supplementary data, Fig. S2) is possible to verify that the presence of functional groups on biochar surface promotes a significant difference in its spectral characteristics. After pyrolysis at higher temperatures, functional groups of biochar surface are lost, which promote a spectral behavior similar to activated carbon. The total content of C, H, N and S were determined using a dry combustion method, the values obtained are based on dry weight of samples and the O content was determined by subtracting from the total mass of the sample (Table 1). The elemental ratios H/C and O/C were used to measure the degree of aromaticity of the samples, illustrated by van Krevelen diagram in Fig. 3. Using more elevated pyrolysis temperatures, biochar samples had higher carbon contents, being registered an increase of 6.44% when compared to the biomass (feedstock) and the biochar obtained at 600 ◦ C. In contrast, it was observed a decrease in oxygen and hydrogen contents, with increasing on pyrolysis temperature. This observation can be related to reactions occurring during this process as the decomposition of organic matter (cellulosic components) from the feedstock, which leads the release of O and H atoms [56]. The H/C and O/C ratios were higher in the samples obtained at lower temperatures and in the biomass. Changes in composition of biochar during pyrolysis process indicates a gradual decrease in the values of H/C and O/C, as well as the increase of aromatic groups (C C) from aliphatic carbon structures, caused by the increase in temperature pyrolysis [1,47].

529

BM

1.4 BC300

1.2 BC400

1.0 0.8

BC600

0.24

0.27

BC500

0.30

0.33

0.36

0.39

O/C Atomic Ratio Fig. 3. Van Krevelen diagram of atomic ratios referring to the biomass and biochar samples. Table 2 Superficial characterization of the biochar samples, activated charcoal (AC) and graphite obtained from B.E.T. analyses. Sample

Surface area (m2 g−1 )

Pore volume (cm3 g−1 )

Pore diameter (Å)

BM BC200 BC300 BC400 BC500 BC600 GRAPH AC

1.28 2.53 1.01 1.10 1.19 1.82 7.15 606

8.33 × 10−4 8.55 × 10−3 1.41 × 10−3 1.51 × 10−3 1.68 × 10−3 3.14 × 10−3 1.53 × 10−2 4.08 × 10−1

26.12 13.51 56.11 55.30 56.42 69.15 85.77 26.90

Measurements performed by B.E.T. method have provided information about porosity and surface area of the samples which are shown in Table 2. In general, there are an increase of surface area and porosity for high temperature of pyrolysis, however, this is not always observed. In the present work, a significant increase of surface area and porosity was not observed which is in agreement with results reported by Ghani et al. [55], wherein the increased surface area does not occur until the pyrolysis temperature had reached 850 ◦ C. Novak et al. [13] reported surface area values of 0.52 and 1.22 m2 g−1 for peanut biochar pyrolyzed at 400 ◦ C and 500 ◦ C, respectively. This suggests that the biochar surface area not necessarily presents a significant variation with the increasing temperature. Based on pore diameter values samples can be classified as microporous, mesoporous and macroporous with diameter less than 2 nm, between 2 and 50 nm and larger than 50 nm, respectively [57]. Thus, the samples were characterized by mesoporous surface, except for the BC200 sample with pore diameter less than 2 nm, is microporous. In contrast, the graphite obtained at temperatures above 600 ◦ C, had surface area and very high porosity with respect to biochar samples. 3.2. Voltammetric performance of CPME modified with biochar for preconcentration of metallic ions Preliminary voltammetric experiments were realized in order to verify the performance of biochar in the preconcentration of lead ions. Fig. 4 presents DPAdS voltammograms obtained using an unmodified CPE after preconcentration step (Curve A) and the CPME with biochar pyrolyzed at 400 ◦ C before (Curve B) and after (Curve C) preconcentration step. DPAdS voltammograms recorded in the potential range of −1.0 V to −0.2 V (vs. Ag/AgCl, KCl 3.0 mol L−1 ) for CPE did not exhibit any significant peak of current after the preconcentration step (Curve A). Measurements performed before the preconcentration step using CPME did not exhibit any faradaic sign related to superficial redox processes in the potential range evaluated (Curve B). However, voltammograms obtained using CPME (Curve C) after preconcentration step has shown an oxidation peak at −0.55 V

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C. Kalinke et al. / Journal of Hazardous Materials 318 (2016) 526–532

Fig. 4. Differential pulse adsorptive voltammograms obtained in acetate buffer solution with CPE after preconcentration step (Curve A) and CPME before (Curve B) and after preconcentration step (Curve C) in solution containing 1.0 × 10−4 mol L−1 Pb2+ ions for 5 min under stirring and at open circuit.

Fig. 5. Differential pulse adsorptive voltammograms using CPME-BC400 (A) and variation of the intensity of the peak current (B) obtained by voltammetric measurements using CPME modified with biochar pyrolyzed at different temperatures, after preconcentration of Pb2+ ( ), Cd2+ ( ) and Cu2+ () ions.

which can be attributed to lead oxidation (Pb0 → Pb2+ + 2e− ). This demonstrates that biochar is able to promote an effective interaction between electrode surface and lead ions, compared to the unmodified CPE. Biochar has properties that make it favorable for the preconcentration of metallic ions, such as the presence of active functional groups and cationic exchange capacity, which enables high adsorption of these ions on their surface [58,59]. After the potentiality of biochar has been verified for preconcentration of Pb(II) ions, additional studies were performed in order to evaluate the voltammetric behavior of biochar samples pyrolyzed at different temperatures in the preconcentration of lead, cadmium and copper ions in concentration of 1.0 × 10−4 mol L−1 , adopting the same parameters as the preliminary study. Faradaic signals between −0.90 V and −0.70 V (vs. Ag/AgCl 3.0 mol L−1 KCl) were attributed to cadmium oxidation previously preconcentrated on the electrode surface (Cd0 → Cd2+ + 2e− ). The oxidation of copper ions (Cu0 → Cu2+ + 2e− ), was confirmed by voltammetric signals obtained in the region close to 0.0 V. Fig. 5A shows the differential pulse voltammograms obtained using CPME modified with biochar samples at 400 ◦ C (CPME-BC400) after preconcentration in presence of Pb2+ , Cd2+ and Cu2+ ions. Fig. 5B shows the variation of the intensity of anodic current peak obtained for the respective ions using CPME modified with biochar prepared at different pyrolysis temperatures. CPME modified with biochar pyrolyzed in intermediate temperatures (400–500 ◦ C) showed higher intensity of anodic current peak. These results demonstrate that these samples still have functional groups that enhance the capacity to adsorb the metallic ions [60,61]. These groups can be identified as acid groups such phenolic, lactonics and carboxylic, present in large quantities according to

Fig. 6. Relation between amounts of Pb2+ ( ), Cd2+ ( ) and Cu2+ () ions adsorbed on biochar and time of preconcentration on the CPME modified with BC400 (A). Pseudo second-order curves for the adsorption kinetics of Pb2+ ( ), Cd2+ ( ) and Cu2+ () ions in CPME-BC400 (B).

obtained results by Boehm titrations (1.63 mEq g−1 for carboxylic, 1.23 mEq g−1 for lactonic and 1.84 mEq g−1 for phenolic groups—for BC400 sample) [40,62]. From 400 ◦ C, biochar has not hemicellulose, and functional groups such as OH− and aliphatic CH are lost, giving rise to aromatics composts less able to interact with the ions present in solution. The CPME with BC400 showed better relative current signal to other electrodes, with anodic peak currents of 144.0 ␮A for the Pb(II) ions adsorption, 70.0 ␮A for Cd(II) and 34.5 ␮A for Cu(II) ions. To estimate the amount of preconcentration ions on the surface of the electrodes, linear sweep voltammetry measurements were performed for the CPME modified with BC400 after preconcentration step (using different times of soaking) on individual solutions containing Pb(II), Cd(II) and Cu(II) ions in concentration of 1.0 × 10−4 mol L−1 . From voltammetric results the area (electric charge) was determined for each ion in order to evaluate the adsorption capacity of biochar. The effect of contact time (t) of the electrodes with the preconcentration solution was evaluated in relation to the quantities of mass adsorbed on the biochar surface, as can be seen in Fig. 6A. The amount of ions adsorbed on biochar is proportional to the contact time with the ions in solution and it increases gradually with increasing time, reaching an equilibrium condition after 20 min of preconcentration time. Furthermore, it was observed that the adsorption of ions occurred in order for adsorption affinity: Pb(II) > Cd(II) > Cu(II). Similar results were reported by other works [34,60,63,64], where biochar had high ability to adsorb lead ions, compared to other metallic ions. This biochar affinity or adsorption capacity can be associated with the size and charge of the ions. With the increase on ionic radius, smaller is the hydrated ionic radius, and an increase in the ability of interaction of these cations with biochar surface is observed. Lead ions have a smaller hydrated ionic radius, and preferably adsorbed for the biochar, however, ions with a greater hydrated radius, such as cadmium and copper may interact with water molecules, thus are less adsorbed on the biochar surface [65]. In order to better understanding the main mechanism involved in the adsorption process of Pb(II), Cd(II) and Cu(II) ions at the electrode surface, the results were submitted to application of the kinetic models. The interaction between ions and the biochar surface was investigated by kinetic models of pseudo-first-order, Eq. (1), and pseudo-second-order, Eq. (2), developed by Weber and Morris [66]. log(qe − qt ) = log(qe ) −

k1 t 2, 303

(1)

C. Kalinke et al. / Journal of Hazardous Materials 318 (2016) 526–532 Table 3 Kinetic parameters obtained for the pseudo-second order model.

References

Cation

k2

R2

qe (␮g g−1 ) present study

literature

Pb(II) Cd(II) Cu(II)

0.266 0.165 0.049

0.978 0.966 0.912

15.94 ± 0.09 4.29 ± 0.13 2.38 ± 0.39

13.58 [63] 3.34 [63] 6.14 [68]

t 1 t = − qt qe k2 q2e

531

(2)

Where, qe is the amount adsorbed in equilibrium time, qt is the amount adsorbed at time t, k1 is a constant of the first order rate and k2 is a constant of the second order rate. The validation of the models was checked by the linear graphs of log (qe − qt ) versus t for the pseudo-first order, and t/qt versus t for pseudo-second order. The curves of the kinetic study suggest that the pseudo-second order model (Fig. 6B) describes more accurately the adsorption results for the evaluated ions, due to higher correlation coefficients (R2 ), and better linearity compared with curves obtained for the pseudo-first order model. This model is based on chemisorption of ions studied at electrode surface assuming that the surface reactions control the adsorption rate [58,64,67]. Moreover, it is observed that lead ions are more easily adsorbed to the biochar surface than other ions. Table 3 present the kinetic parameters of pseudo-second order model, the obtained values of the amount adsorbed qe (␮g g−1 ), the constant rate k2 and the correlation coefficient R2 . In this context, biochar prepared from castor oil cake is comparable with other reported works, and it has a large capacity and ability to interact with ions in solution, which can be mainly explained by highly functionalized surface of the material. Furthermore, the adsorption process may also occur concurrently with other interactions on surface of biochar, such as cation exchange, where the ions in solution can produce an ion exchange with other cations from biochar; and/or complexation of ions with free functional groups, or mineral oxides present on the surface of the biochar [5,11,59,69]. 4. Conclusions Characterization of biochar showed that samples exhibited different physical and/or chemical properties, according to the increase in pyrolysis temperature. The voltammetric evaluation using biochar in CPME demonstrated that material is able to Pb(II), Cd(II) and Cu(II) ions preconcentration which improve the voltammetric performance of the electrode. In this issue, we would like to highlight that BC400 sample promoted the better results for ions preconcentration with higher sensitivity to adsorb metallic ions. Values found for amount adsorbed were 15.94 ± 0.09 ␮g g−1 , 4.29 ± 0.13 ␮g g−1 and 2.38 ± 0.39 ␮g g−1 for Pb(II), Cd(II) and Cu(II) ions, respectively. Furthermore, kinetic models have helped to describe the adsorption process based on pseudo-second order model where the rate of the chemisorption of ions studied at electrode surface control the adsorption rate. Acknowledgements We gratefully acknowledge financial support from Brazilian foundations: CAPES and CNPq. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2016.07. 041.

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